The present invention relates generally to the treatment of process waste streams. In preferred embodiments, the present invention relates to the treatment of process wastewater streams containing hydroxylamine with hydrogen peroxide so as to decompose the hydroxylamine in the stream and thereby render it suitable for biological wastewater treatment.
Excess hydroxylamine (NH2OH) in process waste water streams can potentially be toxic to the biological mechanisms responsible for adequate wastewater treatment. Therefore, excess hydroxylamine must be decomposed prior to biological wastewater treatment.
According to the present invention, decomposition of excess hydroxylamine is accomplished by bringing the hydroxylamine-containing wastewater stream into contact with hydrogen peroxide. Most preferably, the wastewater stream is at neutral pH (e.g., pH between 7-8) and at elevated temperature (e.g., at least about 90xc2x0 C.) when brought into contact with hydrogen peroxide. In another aspect of this invention, the presence of ammonium ion (NH+) accelerates the decomposition of hydroxylamine in the wastewater stream.
These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.
The present invention necessarily depends upon a wastewater stream containing residual amounts of hydroxylamine. It is especially useful in the treatment of wastewater streams that contain greater than about 2.5 mg hydroxylamine per liter since such excess amounts of hydroxylamine are known to be potentially toxic to the biological mechanism responsible for adequate wastewater treatment. However, the present invention may also be usefully employed to treat wastewater streams with residual hydroxylamine content less than about 2.5 mg/liter. The present invention has been found to be especially effective in the treatment of wastewater streams containing greater than about 100 mg hydroxylamine per liter, particularly between about 800 to about 1100 mg hydroxylamine/liter.
Residual amounts of free-base hydroxylamine (NH2OH) in water are readily decomposed by treatment with hydrogen peroxide (H202). The time required for decomposition ranges from 0.5 hour to in excess of 24 hours. Decomposition is either partial or complete depending on the reaction conditions. In this regard, higher temperature (e.g., about 90xc2x0 C. or greater) results in a higher rate of decomposition compared to lower temperature (e.g., about 60xc2x0 C. or greater). Neutral pH (e.g., between about 7-8) results in a significantly higher rate of decomposition compared to alkaline pH (e.g., between about 10-12).
The molar ratio of hydrogen peroxide to hydroxylamine will be greater than about 55%, and more preferably greater than about 68%. Thus, for example, a molar ratio of 58% hydrogen peroxide is sufficient to cause approximately 77% decomposition of the hydroxylamine, while a molar ratio 69% H202:NH2OH is sufficient to cause 97% decomposition of the hydroxylamine. In accordance with the present invention, therefore, the hydroxylamine present in the process stream will be decomposed in a substantial amount of greater than about 75%, and more preferably in an amount greater than about 95%.
The presence of ammonium ion (NH+) increases the rate of decomposition. Specifically, ammonium ion is preferably present in an amount between about 300 to about 500 mg/L, and more preferably between about 400 to about 450 mg/L, at 90xc2x0 C.